The major sources of commercially mined potash have historically come from large sylvite mineral deposits. To date there has been no commercial production of potash fertilisers from glauconite rich deposits. Potassium rich glauconite deposits appear abundant and several efforts have been directed to the production of potash from these resources. Due to a variety of circumstances, each attempt has been commercially unsuccessful. Several authors have reported glauconite to be readily leached in mineral acids, resulting in the extraction of metals present in glauconite to solution. Whilst there have been several flow sheets for the recovery of potassium from these leach liquors proposed, all have proven commercially un-economic.
By way of example, the Moxham process (see the Annual Survey of American Chemistry, Vol. 1, Ed. Hale, W. J., Published for National Research Council by The Chemical Catalogue Company, Inc. N.Y., 1927, pages 85 and 86) involves the hot digestion of greensand in 40-50% sulfuric acid, which dissolves >90% of the potassium, iron, aluminium and magnesium present. The metal sulfates are crystallised by evaporation and/or the addition of concentrated sulfuric acid, and are then thermally decomposed (500° C.) forming Fe2O3 and SO3 for sulfuric acid production. Following water leaching of the calcine, the Fe2O3 is separated from the dissolved K2SO4 and Al2(SO4)3 and K-alum (KAl(SO4)2.12H2O) is crystallised. The K-alum is roasted at 900° C. to produce K2SO4, Al2O3 and SO2 for sulfuric acid recovery. Following water leaching of the calcine, the insoluble Al2O3 is separated and K2SO4 crystallised from solution. This process suffered in particular from high energy costs as a result of the K-alum calcination stage.
Further, the McWhorter process (see for example U.S. Pat. No. 1,843,779) describes the leaching of greensand with excess sulfuric acid, diluted sufficiently to prevent the crystallisation of salts during leaching. High metal extractions can be obtained and the liquor is separated from the silicon rich residue by decantation. The greater part of the iron is present in the ferric form. The crystallisation of FeSO4.H2O is forced by the reduction of iron using a reducing agent such as iron metal. The solubility of FeSO4 decreases above 65° C. so the crystallisation of FeSO4.H2O is conducted at close to the boiling point of the solution. The FeSO4.H2O is separated from the liquor in which K-alum crystallises on cooling. The K-alum is re-crystallised to remove entrained iron. FeSO4.H2O is thermally decomposed (500° C.) forming Fe2O3 and SO2 for sulfuric acid production. The recovery of K2SO4 from K-alum results in a similar fate to that of the Moxham process described above as it requires an expensive calcination step to convert K-alum to K2SO4, Al2O3 and SO3.
The method of the present invention has as one object thereof to overcome substantially the abovementioned problems of the prior art, or to at least provide a useful alternative thereto.
The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to formed part of common general knowledge as at the priority date of the application.
Throughout the specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.